it relates to the shape of a mug which makes it easier to imagine how much water is needed for a cup of tea

facts:

a) kettle is one of the most energy consuming appliances
b) 75% of people in the UK overfill their kettles
c) it costs on average £24 per year to use a kettle
d) 40% of people boil a kettle 5 times a day or more

It’s hard to translate the capacity of a mug into a kettle. As a result we boil more water than we need which means using more electricity, longer boiling time and higher bills.